The invention relates to amplifying electrode tubes operating at microwave frequencies. It applies more particularly to TWTs (travelling wave tubes), and it is therefore with regard to such a tube that the invention will be described. Such tubes are used, for example, for the transmission of telecommunication signals between Earth and satellites. They are also used as power transmitters in radars.
It will be briefly recalled that a TWT is a vacuum tube using the principle of interaction between an electron beam and a microwave electromagnetic wave in order to transfer part of the energy contained in the electron beam to the microwave so as to obtain, as output from the tube, a microwave of higher energy than that of the wave injected into the input of the tube.
FIG. 1 recalls the general principle of a TWT. The TWT shown is a helix TWT, but other types of TWT, such as TWTs with coupled cavities, TWTs with folded waveguides in the form of meanders, etc., are just as well covered by the invention.
TWTs comprise an elongate tubular sheath 10, in which a vacuum is created, with, at a first end, an electron gun 11 that emits an electron beam 12 and, at a second end, a collector 14; the collector collects the electrons that have given up some of their initial energy to the electromagnetic wave that it is desired to amplify. The electron beam 12 is substantially cylindrical over almost the entire length of the tube between the gun 11 and the collector 14 along an axis 15. This cylindrical beam shape is obtained, on the one hand, by the shape of a cathode 16 of the electron gun 11 (a cup-shaped convergent cathode) and, on the other hand, by magnetic focussing means provided over the entire length of the sheath 10 between the exit of the electron gun 11 and the entrance of the collector 14. In the electron gun 11, it is the cathode 16 that emits the electron beam 12. These focussing means comprise, for example, annular permanent magnets 18 that are axially magnetized and of magnetization that alternates from one magnet to the next; these magnets surround the sheath 10 and are separated from one another by pole pieces 20 of high magnetic permeability.
In the case of a helix TWT, the electron beam 12 passes into a helical conducting structure 22 along which the microwave electromagnetic wave to be amplified flows; the amplification of microwave energy takes place by interaction between this microwave and the electron beam 12 that passes through the centre of the helix. The latter serves to decelerate the microwave in such a way that its velocity, along the axis 15 of the electron beam 12, is approximately equal to that of the electron beam 12.
A signal to be amplified of power Pe is injected at one end of the helical conducting structure 22 through a plug and a port 24 inside the sheath 10. An amplified signal of power Ps is extracted at the other end of the helical conducting structure 22 through a plug and a port 26. The amplification gain G of the electron tube is defined by the ratio G=Ps/Pe or, expressed in decibels, 10 log10(Ps/Pe). The efficiency η of the amplification is defined by:η=Ps/VoxIo.Vo represents the voltage between the cathode 16 and the collector 14 and Io represents the current flowing in the cathode 16. The efficiency η is generally around 20 to 30%. It is often called the interaction efficiency ηi and it characterizes that part of the energy of the electron beam 12 converted into microwave energy in the amplified signal. The remaining energy, (1−ηi) VoxIo, in the electron beam 12 after the latter has passed through the helical conducting structure 22, is then dissipated in the collector 14 where the electrons of the beam 12 bombard the walls of the collector 14 and convert their kinetic energy into heat. This heat is then discharged to the outside of the electron tube by conduction, convection or radiation. On the outside of the elongate tubular sheath 10, the electron tube usually has, near the collector 14, a heat sink (not shown in FIG. 1). This heat sink is, for example, cooled by circulation of a liquid or gaseous fluid.
In practice, one portion of the current Io, coming from the cathode 16, flows in the helical conducting structure 22 as shown in FIG. 2.
In this figure, the collector 14 is connected to the positive pole 28 of a DC voltage source 30. The helical conducting structure is also connected to the positive pole 28. The negative pole 32 of the DC voltage source 30 is connected to the cathode 16. The electron beam 12 develops between the cathode 16 and the collector 14. In an experimental arrangement, using a 10 kV DC voltage source 30, a current of 1 A output by the cathode 16 is obtained in the electron beam 12 and a power Ps of 2 kW is obtained as output from the helical conducting structure 22. The return current between the collector 14 and the pole 28 is 0.99 A and the current between the helical conducting structure 22 and the pole 28 is 0.01 A. The efficiency is then expressed as:       η    ⁢                  2        ⁢                                  ⁢        kW                    10        ⁢                                  ⁢        kV        ×                  (                      0.99            +            0.01                    )                      =      20    ⁢                  ⁢          %      .      
The efficiency of an electron tube may be improved by using two voltage sources. This alternative arrangement is shown in FIG. 3. A first DC voltage source 34, for example of 10 kV, is connected between the cathode 16 and the helical conducting structure 22 and a second DC voltage source 36, the voltage of which is lower than that of the first voltage source, for example 6 kV, is connected between the collector 14 and the cathode 16. Assuming the same current and power values as in the example given above in FIG. 2, the efficiency is then expressed as:   η  =                    2        ⁢                                  ⁢        kW                              (                      10            ⁢                                                  ⁢            kV            ×            0.01                    )                +                  (                      6            ⁢                                                  ⁢            kV            ×            0.99                    )                      =          33      ⁢              %        .            
Advantageously, the collector 14 comprises several electrodes raised to various potentials. These various electrodes have the purpose of decelerating the electrons before they strike the walls of the electrodes. Thus, the heat dissipated in the collector 14 is less and the efficiency η increases.
An example of such a collector is shown in FIG. 4. In this example, the 10 kV DC voltage source 34 is connected between the helical conducting structure 22 and the cathode 16. A current of 0.1 A flows in the voltage source 34.
A DC voltage source 38, for example of 6 kV, is connected between a first electrode 40 and the cathode 16. A current of 0.4 A flows in the voltage source 38. A DC voltage source 42, for example of 4 kV, is connected between a second electrode 44 and the cathode 16. A current of 0.48 A flows in the voltage source 42. A second voltage source 46, for example of 1 kV, is connected between a third electrode 48 and the cathode 16. A current of 0.01 A flows in the voltage source 46. The three electrodes 40, 44 and 48, which belong to the collector 14, are placed in such a way that the electrode 40, subjected to the highest voltage relative to the cathode 16, is the closest to the cathode 16 and the electrode 48, subjected to the lowest voltage relative to the cathode 16, is furthest away from the cathode 16. Again assuming the power Ps is 2 kW, the efficiency is expressed in the following manner:   η  =                    2        ⁢                                  ⁢        kW                              (                      10            ⁢                                                  ⁢            kV            ×            0.01                    )                +                  (                      6            ⁢                                                  ⁢            kV            ×            0.40                    )                +                  (                      4            ⁢                                                  ⁢            kV            ×            0.48                    )                +                  (                      1            ⁢                                                  ⁢            kV            ×            0.01                    )                      =          45      ⁢                          ⁢              %        .            